1. Field of the Invention
The present invention relates to a near-field optical head for recording/reading information on a recording medium by utilizing the interaction of a near field light, and to a method for fabricating the same.
2. Background Information
An information recording/reading apparatus using lights has been evolving into the realization of large capacity and downsizing. On that account, the realization of a high density recording bit is demanded. As measures for that, studies using a blue-violet semiconductor laser or an SIL (Solid Immersion Lens) have been conducted. However, it is only hoped that these techniques will result in the improvement to the extent of a few times current recording density due to a problem of the diffraction limit of light. As contrast to this, a method for recording/reading information utilizing a near field light is expected as a technique of handling optical information in microregions exceeding the diffraction limit of light.
This technique utilizes the near field light that is generated by the interaction of a microregion with an aperture formed in a near field optical head having a size equal to or under the light wavelength. Thereby, optical information in areas equal to or below the light wavelength can be handled, which is considered to be a limit in a conventional optical system. Thus, the realization of a high density optical memory can be expected. The principle in reading will be introduced in brief. Generally, in a method called a collection mode, a scattered light first is irradiated onto the surface of a recording medium and thereby a near field light is localized in the periphery of a micromark in accordance with a structure of the micromark on the surface of the recording medium. This near field light is optically interacted with an aperture to be converted into a scattered light and is detected through the aperture and thereby data reading is made possible. Additionally, a method called an illumination mode, a propagation light is irradiated onto the aperture and thereby a near field light is generated in the periphery of the aperture. The near field light is allowed to come close to the surface of the recording medium to interact with the micro-optical information recorded on the surface of the recording medium. The light scattered thereon is detected by a detector that is disposed separately and thereby reading can be conducted. Furthermore, as a method for recording information, it is performed such that a near field light generated from an aperture is irradiated onto the surface of a recording medium to change the topology of the microregion on the recording medium (heat mode recording) or the refractive index or the transmissivity of the microregion is altered (photon mode recording). The near field optical head having an aperture exceeding the diffraction limit of light is used in these methods and thereby the realization of a high density recording bit exceeding an conventional optical information recording/reading apparatus can be attained.
In the case of fabricating such a near field optical head for recording/reading optical information, the aperture formation that directly influences the resolution or the signal-to-noise ratio of signals is an important process. As one method for producing an aperture, a method disclosed in Japanese Patent Publication No. 21201/1993 is known. In the manner of producing the aperture by this method, an opaque film on a point is plastically deformed by pressing a pointed light waveguide on which the opaque film is deposited against a hard flat plate with a very small amount of pressing, which is well-controlled by a piezoelectric actuator.
Additionally, another method for forming an aperture is disclosed in Japanese Patent Laid-Open No. 265520/1999. The manner of forming the aperture by this method is conducted by irradiating an FIB (Focused Ion Beam) to the vicinity of the point of an opaque film covering a projection from the side direction to remove the opaque film from the point of the projection.
However, according to the method of Japanese Patent Publication No. 21201/1993, the aperture can be formed on the light waveguide only one by one. Additionally, a piezoelectric actuator having a moving resolution of a few nanometers is needed to control the amount of pressing and thus an aperture forming apparatus has to be placed in an environment which is little influenced by vibration of other devices or air. Furthermore, it takes much time to adjust a wave guide rod to vertically abut on the flat plate. Moreover, in addition to the piezoelectric actuator having a small moving amount, a mechanical translation platform having a large moving amount is needed. Besides, when the pressing amount is controlled by using the piezoelectric actuator having a small moving resolution, a control unit is required and it takes a few minutes to control and form the aperture. Therefore, for aperture formation, a large-scale apparatus such as a high voltage power supply or a feedback circuit is needed. In addition, a problem has arisen that costs for aperture formation are increased.
Additionally, according to the method of Japanese Patent Laid-Open No. 265520/1999, a fabrication object is the projection on the flat plate. However, since the aperture is formed by using the FIB, the time required to form one aperture is as long as ten minutes. Furthermore, because of using the FIB, a sample needs to be placed in vacuum. Thus, a problem has arisen that fabrication costs for aperture fabrication are increased.